(Interspersed) two-phase flows

The open-source files of this option are:

1. gxidif for inter-fluid diffusive-transfer coefficients
2. gxival for the inter-face values of the solved-for properties
3. gxvirms for the "virtual-mass" sources
4. gximas for the inter-phase mass-transfer rates
5. gxifric for the inter-phase friction coefficients
6. gxlatg for simulating the effect of gravity on the relative motion of two fluids separated by a near-horizontal interface
7. gxvelslp for computing the slip velocity between the two-phase, and putting it in a 3D store for print-out or other display when desired.
8. gx2phs for other utilities.